1. Field of the Invention
The present invention relates to a dense wavelength division multiplexing (WDM)-passive optical network system, and more particularly to a dense WDM-passive optical network system utilizing self-injection locking of Fabry-Perot laser diodes, in which output optical signals of different wavelengths are partially fed back by a partial mirror, so as to injection-lock the Fabry-Perot laser diodes, respectively.
2. Description of the Related Art
Recently, demand for a network architecture based on an optical network has abruptly increased. Such an optical network-based network architecture can provide broadband multimedia services and high-speed and large-capacity Internet services. Interest in an optical network directly connected to optical network units (ONUs), using optical fibers, has also increased in order to provide broadband services to subscribers.
The most economical one of the currently used optical networks is a passive optical network (PON). The PON is a system in which a central office (CO), that is, a service provider, and ONUs, that is, service demanders, are configured only by passive optical elements.
In such a PON, typically, the connection between the central office and a remote node installed in an area adjacent to subscribers is achieved using a trunk fiber, whereas the connection between the remote node and each ONU is achieved using a distribution fiber, in order to minimize the total length of optical lines defined in the PON.
Such a PON has various advantages in that it is possible to reduce the initial installation costs while easily carrying out the maintenance and repair of the PON because the total length of optical lines defined in the PON is minimized, and subscribers share passive optical elements. By virtue of such advantages, use of such a PON is greatly increasing.
In particular, WDM-PON is being highlighted as a next-generation optical network meeting the needs of the information age of the future because it can provide a large quantity of information to each subscriber while maintaining high security levels and easily achieving an improvement in performance. In order to cope with an expansion of services and an increase in the number of subscribers, active research has also recently been conducted into a dense WDM-PON (DWDM-PON) which has a narrow channel gap, so that it has an increased number of channels, thereby being capable of providing a large quantity of information to an increased number of subscribers.
FIG. 1 is a schematic diagram illustrating the configuration of a conventional DWDM-PON system. As shown in FIG. 1, such a DWDM-PON is a system in which different wavelengths are assigned to respective subscribers by a central office so that the central office can simultaneously transmit data to the subscribers through a single optical communication line. Respective subscribers can also transmit data, using different wavelengths assigned thereto, respectively.
In order to assign different wavelengths to respective subscribers, this DWDM-PON should be equipped with light sources respectively adapted to provide different wavelengths corresponding to respective assigned wavelengths. In particular, there is a problem in that transmission nodes should use, as their light sources, expensive light sources such as distributed feedback laser diodes having a very narrow spectrum width, in order to minimize interference between adjacent wavelengths (channels).
Since such a conventional DWDM-PON uses light sources having a very narrow spectrum width, it is also necessary to use an additional device such as a temperature stabilizer or a current stabilizer, in order to stabilize oscillating wavelengths. As a result, there is a problem of high system construction costs. Furthermore, there is a problem in that optical signals of different wavelengths should be uniform because those optical signals must be transmitted to respective subscribers at a narrow wavelength gap.
In order to solve these problems, research has been conducted into economically constructing a DWDM-PON using commercially-available, inexpensive optical elements, and there are some associated research reports.
For example, there is a research report entitled “A low cost WDM source with an ASE injected Fabry-Perot semiconductor laser”, IEEE Photonics Technology Letter, Vol. 12, no. 11, pp. 1067–1069, 2000. This research report discloses a method for economically implementing an optical network system by using an ASE (Amplified Spontaneous Emission) and an inexpensive Fabry-Perot laser diode as respective light sources of a central office and each ONU. In accordance with this method, an ASE outputted from the central office is injected into the Fabry-Perot laser diode of the ONU to lock the output wavelength of the Fabry-Perot laser diode at the same wavelength as that of the ASE (Hereinafter, this operation is referred to as “injection locking”.). As a result, the Fabry-Perot laser diode can oscillate in a single mode, as a distributed feedback laser diode. However, this method has a drawback in that the central office must be equipped with a separate light source for generating an ASE.
There is another research report entitled “Upstream traffic transmitter using injection-locked Fabry-Perot as modulator for WDM access networks”, Electronics Letters, Vol. 38, No. 1, pp. 43–44, 2002. This research report discloses a method for economically implementing an optical network system using a distributed feedback laser diode and a Fabry-Perot laser diode as respective light sources of a central office and each ONU. In accordance with this method, the ONU receives an optical signal outputted from the distributed feedback laser diode to use a part of the received optical signal for signal detection while using the remaining part of the received optical signal for injection locking. However, this method has a drawback in that the distributed feedback laser diode used as the light source of the central office is expensive.